The use of thermal energy to destroy bodily tissue can be applied to a variety of therapeutic procedures, including the destruction of tumors. Thermal energy can be imparted to the tissue using various forms of energy, such as radio frequency electrical energy, microwave or light wave electromagnetic energy, or ultrasonic vibrational energy. Radio frequency (RF) ablation, for example, is effected by placing one or more electrodes against or into tissue to be treated and passing high frequency electrical current into the tissue. The current can flow between closely spaced emitting electrodes or between an emitting electrode and a larger, common electrode located remotely from the tissue to be heated.
One disadvantage with these techniques is that maximum heating often occurs at or near the interface between the therapeutic tool and the tissue. In RF ablation, for example, the maximum heating can occur in the tissue immediately adjacent to the emitting electrode. This can reduce the conductivity of the tissue, and in some cases, can cause water within the tissue to boil and become water vapor. As this process continues, the impedance of the tissue can increase and prevent current from entering into the surrounding tissue. Thus, conventional RF instruments are limited in the volume of tissue that can be treated.
Fluid enhanced ablation therapy, such as the SERF™ ablation technique (Saline Enhanced Radio Frequency™ ablation), can treat a greater volume of tissue than conventional RF ablation. The SERF ablation technique is described in U.S. Pat. No. 6,328,735, which is hereby incorporated by reference in its entirety. Using the SERF ablation technique, saline is passed through the needle and heated, and the heated fluid is delivered to the tissue immediately surrounding the needle. The saline helps distribute the heat developed adjacent to the needle and thereby allows a greater volume of tissue to be treated with a therapeutic dose of ablative energy. The therapy is usually completed once a target volume of tissue reaches a desired therapeutic temperature, or otherwise receives a therapeutic dose of energy.
However, it can be challenging to determine with precision when a particular targeted volume of tissue has received the desired therapeutic dose of energy. For example, Magnetic Resonance Imaging (MRI) can be used during ablation therapy to monitor the extent of the developing treatment zone, but MRI is often prohibitively costly for this type of procedure. Ultrasonic imaging can also be used, but does not reliably or accurately depict the volume of the treatment zone.
Furthermore, while fluid enhanced ablation therapy generally creates treatment zones in tissue surrounding an ablation device that are spherical in shape, anatomical features and differences in tissue types can result in non-uniform propagation of the treatment zone. Accordingly, in some cases it can be desirable to correct for a developing non-uniform treatment zone that results from anatomical features in the targeted volume of tissue (e.g., a nearby blood vessel that is moving heat away from a treatment zone). Moreover, in some situations it can be desirable to create a treatment zone having a non-standard shape. Corrective or other shaping of the developing therapy treatment zone cannot be accomplished, however, without accurate measurements of the temperature in tissue surrounding the ablation device.
Accordingly, there remains a need for devices and techniques for more accurately and reliably monitoring the temperature of tissue during fluid enhanced ablation therapy.